In recent years, lithium secondary batteries have been widely used as driving power supplies for small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode comprised of a lithium compound oxide, a negative electrode comprised of a carbon material or a lithium metal, and a nonaqueous electrolytic solution. As the nonaqueous electrolytic solution, carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are used.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode material brings about a reduction of the battery performance, when part of the solvent of the nonaqueous electrolytic solution locally undergoes an oxidative decomposition during the charging, because the decomposition products inhibit the desired electrochemical reaction of the battery. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolytic solution.
Also, a lithium secondary battery using, for example, a highly crystallized carbon material, such as natural graphite or artificial graphite, as a negative electrode material brings about a reduction of the battery performance, when the solvent of the nonaqueous electrolytic solution undergoes a reductive decomposition on the surface of the negative electrode during the charging. Even in the case of EC, which is widely used as a solvent for the nonaqueous electrolytic solution, it partly undergoes reductive decomposition during repeated charging and discharging cycles, resulting in reduction of the battery performance.
Particularly, decomposition of the nonaqueous electrolytic solution on the positive electrode and the negative electrode causes gas generation, which leads to expansion of the battery and impairs mobility of lithium ions as a result of trapping of the gas between the electrodes, thereby reducing the battery performance.
Techniques for improving the battery characteristics of such lithium secondary batteries are known, for example, in Patent Documents 1 to 5. Patent Document 1 discloses a nonaqueous electrolytic solution containing 0.05% to 10% by volume of glycol sulfite (also referred to as ethylene sulfite) as a cyclic sulfite. However, this document does not describe the retention of capacity at high temperatures.
Patent Document 2 discloses a nonaqueous electrolytic solution containing erythritan sulfite as a cyclic sulfite. However, in Examples of this document, merely 3 cycles are tested, and cycle property, for example, after 100 cycles is not evaluated. This document also does not describe cycle property in a high-temperature environment.
Patent Document 3 discloses a nonaqueous electrolytic solution containing ethylene sulfite, erythritan sulfite and the like to improve cycle property. However, this nonaqueous electrolytic solution exhibits a low charging and discharging efficiency and insufficient characteristics in a high-temperature environment.
Patent Document 4 discloses a nonaqueous electrolytic solution containing ethylene sulfite and vinylene carbonate. In an optimum Example, this document describes that the retention of capacity after 100 cycles at 45° C. is 90.0%. However, a nonaqueous electrolytic solution having a higher retention of capacity is still desired.
Patent Document 5 discloses a nonaqueous electrolytic solution containing 1,3,2-dioxathiolane-2,2-dioxide and the like as a sulfate compound for lithium secondary batteries and describes excellent storage property of the battery which is left to stand at high temperatures in a charged state. In this nonaqueous electrolytic solution, the battery characteristics are improved to some extent but are still far from satisfaction. Nonaqueous electrolytic solutions and lithium secondary batteries having further improved characteristics are needed.                [Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-120837        [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2000-188127        [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2002-270230        [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2002-25611        [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2004-185931        